The proposed research includes a range of projects that are linked by a common experimental approach, theoretical framework and general goal. The goal is to advance our understanding of the functional organization of the visual system by tracing the flow of information through it. The theoretical framework is basically that of systems engineering, with standard notions of nonlinearity, sampling, filtering and signal and noise taking an important role. The experimental approach is exclusively psychophysical, involving investigations of visual performance and visual phenomena. As in previous phases of the project, one central concern is the question: Why isn't vision perfect? Adaptive optics allows optical losses to be largely bypassed. We plan to use it to investigate the role of retinal sampling and filtering, by the photoreceptor mosaic and by postreceptoral neural arrays, in limiting spatial resolution for such stimuli. We will use adaptive optics to visualize the S cone mosaic in the intact eye. New experiments to evaluate the effect of chromatic defocus on image quality are needed to support the quantitative correspondence between image quality, sampling density and resolution. Indeed, the very idea that sampling limits resolution needs more careful experimental and theoretical scrutiny than it has yet received. Some of the proposed projects exploit visual nonlinearity of one kind or another to track the loss of either spatial or temporal visual resolution along the visual pathway from retina to perception. An example is our finding that spatial filtering is adaptively nonlinear, trading resolution for the benefits of noise averaging primarily when contrast is low. Our evidence indicates that spatial and temporal resolution losses are distributed, with progressively narrower passbands as signals penetrate the system. We plan to follow up on our observation that flicker that is too fast to be perceived can nevertheless reduce visual sensitivity, a finding of interest for display design as well as for its implication that intermediate stages of neural processing represent flicker that is not consciously perceived.